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First published online January 11, 2006; 10.1104/pp.105.067918 Plant Physiology 140:548-557 (2006) © 2006 American Society of Plant Biologists The Regulation of DWARF4 Expression Is Likely a Critical Mechanism in Maintaining the Homeostasis of Bioactive Brassinosteroids in Arabidopsis1Department of Biological Sciences, College of Natural Sciences, Seoul National University, Seoul 151–747, Korea (H.B.K., M.K., C.S.A., I.L., S.C.); Division of Molecular and Life Science, Pohang University of Science and Technology, Pohang 790–784, Korea (H.R., I.H.); RIKEN, Wako-shi, Saitama 351–0198, Japan (S.F., S.Y.); and Department of Chemistry, Joetsu University of Education, Joetsu-shi, Niigata 943–8512, Japan (S.T.)
Mutants that are defective in brassinosteroid (BR) biosynthesis or signaling display severely retarded growth patterns due to absence of growth-promoting effects by BRs. Arabidopsis (Arabidopsis thaliana) DWARF4 (DWF4) catalyzes a flux-determining step in the BR biosynthetic pathways. Thus, it is hypothesized that the tissues of DWF4 expression may represent the sites of BR biosynthesis in Arabidopsis. Here we show that DWF4 transcripts accumulate in the actively growing tissues, such as root, shoot apices with floral clusters, joint tissues of root and shoot, and dark-grown seedlings. Conforming to the RNA gel-blot analysis, DWF4:
1 This work was supported by grants from the Plant Diversity Research Center of the 21st Century Frontier Research Program, funded by Ministry of Science and Technology of the Korean government (grant no. PF0330201–00 to S.C.), the Plant Metabolism Research Center at Kyung Hee University, the Science Research Center Program from the Korea Science and Engineering Foundation (KOSEF; to S.C.), and the Plant Signaling Network Research Center of KOSEF, Korea University (to I.H.). H.B. Kim and M. Kwon were supported by a Brain Korea 21 Research Fellowship from the Korean Ministry of Education and Human Resource Development. 2 These authors contributed equally to the paper. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Sunghwa Choe (shchoe{at}snu.ac.kr). Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.067918. * Corresponding author; e-mail shchoe{at}snu.ac.kr; fax 82–2–872–1993. Received July 1, 2005; returned for revision December 19, 2005; accepted December 19, 2005. This article has been cited by other articles:
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-glucuronidase (GUS) histochemical analyses more precisely define the tissues that express the DWF4 gene. Examination of the endogenous levels of BRs in six and seven different tissues of wild type and brassinosteroid insensitive1-5 mutant, respectively, revealed that BRs are significantly enriched in roots, shoot tips, and joint tissues of roots and shoots. In addition, DWF4:GUS expression was negatively regulated by BRs. DWF4:GUS activity was increased by treatment with brassinazole, a BR biosynthetic inhibitor, and decreased by exogenous application of bioactive BRs. When DWF4:GUS was expressed in a different genetic background, its level was down-regulated in brassinazole resistant1-D, confirming that BRASSINAZOLE RESISTANT1 acts as a negative regulator of DWF4. Interestingly, in the brassinosteroid insensitive2/dwf12-1D background, DWF4:GUS expression was intensified and delocalized to elongating zones of root, suggesting that BRASSINOSTEROID INSENSITIVE2 is an important factor that limits DWF4 expression. Thus, it is likely that the DWF4 promoter serves as a focal point in maintaining homeostasis of endogenous bioactive BR pools in specific tissues of Arabidopsis. 
